Caching added to #articles and #sorted_articles

Added caching for #articles and #sorted_articles to speed up for may items.
only cache when preprocessing is ready

Only cache these values if preprocessing is ready. Preprocessing is ready when the
@items attribute is frozen.
removed unneccessary line for returning instance variable
removed doubled block for #articles
removed doubled block for #sorted_articles
Merge pull request #269 from matthiasbeyer/blogging-helper-caching

Cache #articles and #sorted_articles